In some applications, such as motor control and switching regulators, a source of low DC voltage is needed in which the DC voltage is not required to be isolated from the AC line. In the past the low DC voltage was obtained by dropping a large percentage of the line voltage across a device such as a resistor after the AC input was converted to filtered DC. This approach was very inefficient. Another common approach was to use a transformer to drop the level of the AC voltage before converting to filtered DC. This approach was expensive because of the cost of the transformer.
Additionally, previous inventions have used transistors that conducted nearly continuously even with large voltage drops across them. Another approach used an SCR which was controlled by an integrating circuit. The charging time of the integrator was controlled by the DC output voltage. While this approach was efficient, it had other problems. During power turn on when the output voltage was low, the SCR fired before the AC input could reach peak value. This lead to a high current surge and high DC output voltage unless additional protection was provided.
A solution to this invention was taught in U.S. Pat. No. 4,709,322 issued to Mirow on Nov. 27, 1987. Mirow taught a supply wherein power was switched in accordance with a comparison between the input and output voltages. When the input was substantially equal to the output, the switch was closed thereby preventing a current surge. However, in the supply of Mirow variations in the input supply voltage could limit the available DC output current. Another problem with the converter of Mirow was that the voltage across the switch at which it may close was not adjusted to compensate for variations in DC output current. This causes loose DC output voltage regulation. In addition the voltage transients caused by the application of the AC input voltage could cause the switch to close at the wrong time.